An organic-inorganic composite that an inorganic component is compounded in an organic polymer (which is sometimes called organic-inorganic hybrid or organic-inorganic composite polymer) has drawn attention as an industrial material with new properties that are both of the characters of the organic polymer and the inorganic component.
Particularly when an organic-inorganic composite is produced to be used for a plastic lens with a high refractive index, use of titanium oxide as the inorganic component is considered to be favorable.
An example of the organic-inorganic composite produced with a titanium oxide sol is disclosed in Japanese Patent Laid-open No. Hei 8-157735 which describes a process for the preparation of an organic-inorganic composite polymer, characterized by hydrolysis and dehydration condensation of a metal alkoxide with an acid or a base as a catalyst in the presence of an organic monomer, followed by polymerization of an organic-inorganic composite containing the obtained metal oxide and the organic monomer.
Japanese Patent Laid-open No. Hei 11-14949 describes a process for producing contact lenses by polymerization of a monomer mixture that contains Component A, a monomer composition containing at least a monomer and a metal oxide having optional organic groups and produced by hydrolysis and dehydration condensation of a metal alkoxide in the presence of the monomer, and Component B, a monomer capable of polyaddition or polycondensation with the monomer composition of Component A.
As processes for producing transparent, homogeneous metal oxide sols, for example, Japanese Patent Laid-open No. Hei 10-298769 describes a process for the preparation of a metal oxide precursor sot, characterized in that water is added to the metal alkoxide at −20° C. or lower in a method for producing a metal oxide precursor sol by hydrolyzing and polymerizing one or more metal alkoxides.
It is known, as a process for forming a thin metal oxide film on the surface of a base plate by a sol-gel method, that a metal alkoxide used as a starting material is hydrolyzed and polymerized to prepare a metal oxide precursor sol and the obtained sol is coated on the surface of a base plate to form a thin film of the metal oxide gel on the surface of the plate, then the gel film is heated at an appropriate temperature. Of these methods, a preferable example is a process that an organic compound able to coordinate to multiple sites is added to stabilize a metal alkoxide so as to control the hydrolysis rate for producing a filmable sot, and the sot is used to form a film according the aforementioned method. Actual examples are that a β-diketone is effective for forming an alumina thin film using aluminum s-butoxide as a starting material [Journal of the Ceramic Society of Japan, 97, 369 (1989)]; and for forming a titania thin film using titanium isopropoxide as a starting material, 1,3-butan-diol is effective [Sumio Koshiba, Toyohashi University of Technology, Doctoral Thesis, March, 1993]; and a β-diketone is effective [Journal of the Ceramic Society of Japan, 97, 213 (1989)]. For forming a zirconia thin film using zirconium n-butoxide as a starting material, use of diethylene glycol is reported to be effective [Journal of the Ceramic Society of Japan, 95, 942 (1987)]. Furthermore, there are reports that use of β-diketones or alkanolamines is effective for the syntheses of composite oxides, such as PbTiO3 and Pb(Zr,Ti)O3, in Journal of American Ceramic Society, 74, 1407 (1991) and Journal of the Ceramic Society of Japan, 98, 745 (1990).
Processes for the preparation of oxide films that make use of hydrolyses of various inorganic salts, such as chlorides, sulfates, nitrates and ammonium salts, and aqua complexes are reported in Physics of Tin Film, 5, p 87 (1969), Academic Press. In addition, Journal of the Ceramic Society of Japan, 102, 200 (1994) describes use of indium nitrate and tin chloride, instead of a metal alkoxide, to prepare an In2O3—SnO2 sot as a composite oxide.
However, in either case of Japanese Patent Laid-open No. Hei 8-157735 or 11-14949, a metal alkoxide is hydrolyzed and dehydration condensed with an acid or a base, and the solvent, water, acid or base is distilled out for bulk polymerization. It is difficult to completely remove the used water, acid or base. Therefore, a problem was the effect of these remaining substances on the polymerization reaction. Particularly when solution polymerization is carried out in an organic solvent, it is necessary to use an acid, a base or a dispersion stabilizer to make the product of the metal alkoxide hydrolysis existing stably in a solution. Therefore, a problem was that these inhibit the polymerization or give bad effects on the physical properties of the product. Furthermore, the said organic-inorganic composite prepared with a titanium oxide gel generally tends to have lower transmittance than that of inorganic-organic composites produced with other metal oxide gels. This fact suggests that titanium oxide aggregates at the stage of concentration following the hydrolysis and dehydration condensation.
The transparent, homogeneous metal oxide sols disclosed in Japanese Patent Laid-open No. Hei 10-298697 also have the same problem as those described above because the pH is in the acidic region.
According to the said process for stabilizing a metal alkoxide by adding a multidentate compound so as to control the hydrolysis rate of the metal alkoxide, a homogeneous sol for forming a film is easily prepared. However, many organic substances that are hard to decompose due to their high boiling points coexist in the sol or gel film. As a result, it is necessary to heat the gel film at a temperature as high as about 500° C. in order to remove the organic substances. Because of many organic substances remaining in the gel film, a heat treatment of the gel film greatly reduces the film weight. In other words, the removal of the organic substances from the gel film creates many pores in the film, being a cause of defects of the obtained thin metal oxide film. There was a problem that various characteristics of the metal oxide, such as mechanical, optical and electric, were not fully displayed. On the other hand, the removal of the pores in the film needs extra energy to make the thin film dense. As described above, the processes using metal salts are basically thermal decomposition methods that cause many problems in film qualities after heat treatment.
It is an object of the present invention to provide a process for the preparation of a thin metal oxide film at a temperature as low as 200° C. or below, and a metal oxide sol suitable for producing a homogeneous organic-inorganic composite, as well as a thin metal oxide film and organic-inorganic composite that have various functions, particularly an organic-inorganic composite with a high refractive index and high transparency.